See the following page:
http://www.9rays.net/asp.net_2/treeview/miscpages/doc/ClientEvents.aspx
And the OnDropJavascript section.
You need to handle this event, and add some code to the handler. Like this:
.....
function MyDropJavascriptHandler()
{
// flytreeview_dragObject.value - is a dragged node
flytreeview_dragObject.value.setPopulateNodesOnDemand(true);
// see http://www.9rays.net/asp.net_2/treeview/miscpages/doc/ClientObjects.aspx for additional client side functions
}